Trona ore is a mineral comprising sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O). It is found in a number of deposits around the world, i.e. in USA, Brazil, China, Egypt, Kenya, India, Tanzania, Turkey. A vast deposit of mineral trona is found in southwestern Wyoming (USA) near Green River. This deposit includes beds of trona and mixed trona and halite (rock salt or NaCl) which covers approximately 2,600 km2. The major trona beds range in size from less than 428 km2 to at least 1,870 km2. By conservative estimates, these major trona beds contain about 75 billion metric tons of ore. The different beds overlap each other and are separated by layers of shale. The quality of the trona varies depending on its particular location in the stratum.
A typical analysis of the trona ore mined in Green River is as follows:
TABLE 1ConstituentWeight PercentNa2CO343.6NaHCO334.5H2O (crystalline and free moisture)15.4NaCl0.01Na2SO40.01Fe2O30.14Insolubles6.3
The sodium sesquicarbonate found in trona ore is a complex salt that is soluble in water and dissolves to yield approximately 5 parts by weight sodium carbonate (Na2CO3) and 4 parts sodium bicarbonate (NaHCO3), as shown in the above analysis. The trona ore is processed to remove the insoluble material, the organic matter and other impurities to recover the valuable alkali contained in the trona.
The most valuable alkali produced from trona is sodium carbonate. Sodium carbonate finds major use in the glass-making industry and for the production of baking soda, detergents and paper products.
A common method to produce sodium carbonate from trona ore is known as the “monohydrate process”. In that process, crushed trona ore is calcined into crude sodium carbonate which is then dissolved in water. The trona insolubles are removed, and the resulting water solution is purified and fed to a crystallizer where pure sodium carbonate monohydrate crystals are crystallized. The monohydrate crystals are separated from the mother liquor and then dried into anhydrous sodium carbonate. However, the soluble impurities contained in the trona ore, tend to accumulate into the crystallizer. To avoid build up of impurities, the mother liquor must be purged. The purge liquor, which represents important quantities for industrial monohydrate plants, is commonly sent to evaporative ponds.
On the other side, sodium bicarbonate is a product with a wide range of interesting properties and a very wide range of applications from the pharmaceutical industry to the human food and animal feed, and to the use in flue gas treatment. In flue gas treatment sodium bicarbonate is most likely among the most efficient chemicals for the removal of a wide range of pollutants (most notably the acidic one), and its use is limited only by the competition of less efficient but much cheaper chemicals such as lime or even limestone.
The production of sodium bicarbonate is currently almost entirely made by the carbonation of sodium carbonate. The carbonation may be made with CO2 from lime kilns or calciners; it can also be done with geological CO2.
Because of the nature of this most important process for the bicarbonate production, the price for bicarbonate is above the price of the soda ash. With such economics the uses of bicarbonate will always be limited by the competition of cheaper substitutes, most notably in the flue gas treatment.
WO 2010072793 (Solvay SA) the content of which is incorporated by reference discloses a process for the joint production of sodium carbonate and sodium bicarbonate using selective dissolution of sodium carbonate while recrystallizing in the same step crude sodium bicarbonate particles comprising the trona insolubles. The process can be operated either in a batch (discontinuous) mode or in a continuous mode. On continuous mode, one or two leaching tanks in series are operated with 15 to 40 minutes mean residence time in each tank. Though the produced crude sodium bicarbonate is fine with mean diameter solid particles size from 7 to 30 μm.
U.S. Pat. No. 4,654,204 discloses a process for the continuous production of purified sodium bicarbonate by introducing solid sodium carbonate (Na2CO3), sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O), and/or Wegscheider's salt (Na2CO3.3NaHCO3) into a reversion slurry. In that process the sodium carbonate from the reversion liquor is bicarbonated with carbonic acid (CO2) in a carbonator and recycled to the reversion slurry to form sodium bicarbonate from fed sodium carbonate. The document is silent on insoluble impurities of the solid feed.
U.S. Pat. No. 4,478,599 discloses a process for continuously controlling the formation of crystal fines in a continuous process for crystallizing sodium bicarbonate, using carbon dioxide gas and a feeding brine by adding a flocculant polymers or copolymers in the mother liquor before the crystallization of sodium bicarbonate. In one of the described process (FIG. 2) using a multi-stage process with a first carbonator zone using CO2 crystallizing sodium bicarbonate, and a second stage crystallizing zone in a cooled reactor, the document teaches that the flocculent is to be added prior to the reception of the mother liquor before the second crystallization stage, as if the flocculant were to be added to the mother liquor in the second crystallization stage then fines will have already formed to such extent that fines control cannot be achieved. The sodium bicarbonate crystallization is operated either with an evaporative crystallizer, or with a cooling crystallizer, or by bicarbonation of the feeding brine with carbon dioxide gas. In this process the raw materials are either in liquid or gas form. The document is silent on a selective dissolution of sodium carbonate from trona and the reversion crystallization of sodium sesquicarbonate into bicarbonate.
U.S. Pat. No. 3,852,427 discloses a process for making sodium bicarbonate spherulites from sodium sesquicarbonate comprising preparing an aqueous solution of sodium sesquicarbonate totally dissolved, introducing an alkali metal phosphate in the aqueous solution, carbonating in batch mode the solution with carbon dioxide and cooling gradually the slurry while continuing the carbonation to obtain the spherulites.
The invention aims at producing bicarbonate from trona in a continuous mode in a smooth and inexpensive way, with an improved process for obtaining coarse sodium bicarbonate crystals and at the same time valorizing the sodium carbonate contained in the trona without the use of total dissolution of trona which uses water that will have to be removed, for instance by evaporation, with the associated energy costs and CO2 foot-print when using fossil energy raw material.